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Biological reactions are the reactions that occur in living things. For example, when we eat food we need to digest it. Without enzymes in our bodies, digestion could take weeks instead of a matter of hours. Enzymes are specific for a certain reaction, for example, the enzyme lactase will only work in reactions where the sugar lactose is present.
Each enzyme, for example lactase, has an active site where the substrate molecules bind to the enzyme. Each enzyme’s active site has the right “shape” for a certain substrate molecule but no other. This is like a jigsaw puzzle where only certain pieces fit together.
Enzymes also have temperatures and pH levels which are optimum for them.